You’ve decided to convert from standard shadow passwords to MD5 passwords. You make the appropriate changes to the /etc/pam.d/ files. What do you do next?

A.
Nothing, the passwords will be changed as users login and out.

B.
Nothing, users will be automatically prompted to change their passwords at the next login.

C.
You need to manually change all the passwords using the passwd program.

D.
Delete and recreate all the users.

E.
Change the /etc/pam.d files back because shadow passwords and MD5 passwords are incompatible.

Explanation/Reference:
The reasoning behind password hashing is that NO ONE can calculate the password from the hash, that means that if you change the hash algorithm from, say, MD5 to SHA512 every user has to manually reset her password.